Question

1. herbicides are chemicals used to kill or inhibit the growth of unwanted plants and weeds. many herbicides work by blocking electron carriers, or reducing their ability to carry electrons, in the light dependent reactions. one such chemical is dcmu (3-(3,4-dichlorophenyl)-1,1-dimethylurea). dcmu is an algaecide and herbicide that works by blocking the electron binding site of plastoquinone. this means that photosystem ii is unable to transfer electrons from the splitting of water to plastoquinone. this effectively shuts down the linear flow of electrons in the light dependent reactions. a) identify the molecule produced in the light dependent reactions that would be most affected by the shutting down of linear electron flow. b) connect how the molecule affected in part (a) is used in the calvin cycle. c) describe what would effectively happen to the calvin cycle as a result of exposure to dcmu?

Explanation:

• a) In the light - dependent reactions, ATP and NADPH are produced. When linear electron flow is shut down (as in the case of DCMU blocking electron transfer), the production of NADPH is most affected. This is because the linear electron flow is directly involved in the reduction of \(NADP^{+}\) to NADPH.
• b) In the Calvin cycle, NADPH is used as a reducing agent. It donates electrons to the \(3 -\)phosphoglycerate (PGA) molecules. Specifically, in the reduction phase of the Calvin cycle, NADPH helps convert PGA to glyceraldehyde - 3 - phosphate (G3P). The electrons from NADPH are used to reduce the carbon atoms in PGA, and the hydrogen atoms from NADPH are also incorporated into the molecules.
• c) Since NADPH (and also ATP, but NADPH is more directly related to the linear electron flow disruption here) is required for the Calvin cycle, the Calvin cycle would slow down or stop. Without NADPH (and ATP), the reduction phase of the Calvin cycle (conversion of PGA to G3P) cannot occur efficiently. As a result, the synthesis of glucose (the end - product of the Calvin cycle, when multiple G3P molecules are combined) would be severely impaired.

Answer:

a) NADPH
b) NADPH is used as a reducing agent in the Calvin cycle. It donates electrons and hydrogen atoms to \(3 -\)phosphoglycerate (\(PGA\)) during the reduction phase to convert it to glyceraldehyde - 3 - phosphate (\(G3P\)).
c) The Calvin cycle would slow down or stop because NADPH (and ATP, to some extent) is required for the reduction phase of the Calvin cycle. Without these molecules, the conversion of \(PGA\) to \(G3P\) (a key step in the Calvin cycle) cannot occur efficiently, and thus glucose synthesis (from \(G3P\)) would be impaired.